1. Field
Example embodiments relate to an optical reference standard and in particular to an optical reference standard usable in the calibration of an optical analysis instrument.
2. Description of the Related Art
Optical reference standards are widely used in optical spectroscopy in order to ensure the correct performance of an analysis instrument. This is typically achieved through the comparison of measurements of optical properties of the optical reference standard performed on one instrument (so called ‘slave’ instrument) with similar measurements performed on the same optical reference standard by a reference instrument (so called ‘master’ instrument) of the same type. Moreover it is known to carry out similar comparative measurements between slave instruments or on the master instrument using the same optical reference standard for each measurement of the comparison.
When using a model, such as a regression model, derived either on the master instrument or using data from several instruments then the accuracy of the slave instrument in determining properties or constituents of a material to be analysed, such as a feed or foodstuff (including any starting or intermediate products), depends upon the accuracy with which the wavelengths and associated radiative intensities in a wavelength range particular to the material can be determined. In order to provide an accurate calibration of the instrument it is therefore important that the optical reference standard has optical characteristics in the wavelength range of interest that are close to the material to be analysed. It is also important in this respect that the reference standard has optical scattering characteristics similar to the material to be analysed.
Moreover, there are often large geographical distances between instruments and attendant time lags between measurements done on the master and the slave(s). A typical instrument standardisation involves making a standardisation measurement on the master instrument, transporting the results of this and the optical reference standard to the slave where the measurements are repeated, and finally transporting the reference standard back to the master where a confirmation measurement is done and a comparison is made with the original standardisation measurement in order to ensure the validity of any associated instrument standardisation. It is therefore also important that the optical reference standard has both mechanical and temporal stability sufficient for the optical properties of the standard to remain substantially constant (that is differences less than what are expected to occur between instruments), at least between measurements by the master.
It is known from U.S. Pat. No. 4,866,644 to provide a reference standard consisting of the particular feed or foodstuff to be analysed. Whilst, in the case of dry feed and foodstuffs, the so produced optical reference standard may have sufficient mechanical and temporal stability this is not the case for ‘wet’ feed and foodstuffs (i.e. solid products having a significant water or other edible-liquid content, such as vegetable oils, or fat content). Such wet products normally undergo rapid chemical changes and are mechanically relatively unstable. Thus, even with the careful and rapid transportation of optical reference standards consisting of such material, it is likely that changes will occur in its optical properties between measurements. This renders such optical reference standards essentially useless.
However, there presently exists a desire to provide an optical reference standard using such natural products so as to likewise provide an effective reference standard for wet feed and foodstuffs. Moreover, where analysis instruments are deployed in-line in a feed or foodstuff production process any contamination problems caused if the line becomes exposed to the optical reference standard may be significantly reduced if the standard comprises such natural components as are typically found in feed and food.
It is an aim of the present invention to provide an optical reference standard for wet products in which at least some of the problems associated with the known standards are alleviated.
Accordingly, there is provided an optical reference standard including an optical reference material, wherein the optical reference material may include a binder and a cereal formed into a matrix to fixedly hold an amount of water, the reference material being constituted with the binder, the cereal and the water present in amounts to provide the reference standard with desired spectral, mechanical and temporal characteristics. The matrix formed by one or both of the binder and the cereal, which in the present context includes mixtures of different cereal crops, can hold the water spatially fixed so as to provide a reference standard, particularly for wet feed or foodstuffs, having enhanced mechanical and temporal stability.
Preferably the cereal employed is one or more of the group wheat, barley, oats and maize. More preferably the cereal employed is oats as this has been discovered by the inventors to provide an optical reference standard having particularly good mechanical and temporal stability.
Usefully, the amount, the constituency and/or the composition of the cereal can be varied together with or independently of the liquid content so as to provide an optical reference standard having scattering and/or spectral response properties that closely match the product to be analysed using the analysis instrument. In this way a better calibration of the analysis instrument can be performed.
The binder employed can usefully be a gelling agent, such as gelatin, that can advantageously provide mechanical stability and throughout which can be dispersed some or all of the water to thereby hold it fixedly located.